MimeMessage.getAllHeaders() returns two different classes in two different projects

Question

I have a project about mailing and works without problem but I created another project that is also about mailing generated MimeMessage cannot be parsed in first project.

I studied the classes and found difference between header class of MimeMessage Both projects are using

javax.mail.internet.MimeMessage

but I logged header as:

Enumeration enumer = originalUserMessage.getAllHeaders();
while(true){
    try{
        Object obj = enumer.nextElement();
        if(obj == null) {
            break;
        }
        LOGGER.info("Header class: "+obj.getClass().getName());
        LOGGER.info("Header super class:" + obj.getClass().getSuperclass().getName());
    } catch (NoSuchElementException nsee) {
        break;
    }

}

first project printed:

2015-03-03 16:57:16 INFO  SmimeWriter:204 - Header class: 
javax.mail.Header
2015-03-03 16:57:16 INFO  SmimeWriter:205 - Header super class: 
java.lang.Object

second project printed

2015-03-03 16:57:39 INFO  SmimeWriter:204 - Header class: 
javax.mail.internet.InternetHeaders$InternetHeader
2015-03-03 16:57:39 INFO  SmimeWriter:205 - Header super class: 
javax.mail.Header

all relevant classes seems like in mail.jar. I am using maven to get jars and both projects getting that jar as:

<dependency>
  <groupId>javax.mail</groupId>
  <artifactId>mail</artifactId>
  <version>1.4.2</version>
</dependency>

what can be the problem. is there any other dependent jars?

both projects are using same method. after code block below other parts of mime message are set but problem occurs here. code:

        MimeMessage originalUserMessage = new MimeMessage(session);

        originalUserMessage.setFrom(from);
        originalUserMessage.setRecipients(Message.RecipientType.TO, to);
        if(cc != null && cc.length>0){
            originalUserMessage.setRecipients(Message.RecipientType.CC, cc);
        }

        Enumeration enumer = originalUserMessage.getAllHeaders();
        while(true){
            try{
                Object obj = enumer.nextElement();
                if(obj == null) {
                    break;
                }
                LOGGER.info("Header class: "+obj.getClass().getName());
                LOGGER.info("Header super class: "+obj.getClass().getSuperclass().getName());
            } catch (NoSuchElementException nsee) {
                break;
            }

        }

Answer 1

This isn't a problem at all. The contract of the MimeMessage.getAllHeaders() method is clear:

Return all the headers from this Message as an enumeration of Header objects.

That documentation is referring to the javax.mail.Header class.

So when you call MimeMessage.getAllHeaders(), it is only safe to assume that the Enumeration will return instances of javax.mail.Header. It might or might not return objects which are a subclass of Header, which according to the basic rules of polymorphism, is a perfectly valid thing to do, as long as the calling code can safely treat them like Header instances.

Depending on how you run your program, the version of JavaMail in your pom.xml may be irrelevant. In Java EE containers (like GlassFish and JBoss) and servlet containers (like Tomcat), JavaMail is provided by the container, so even if you package your own version of JavaMail with your application, the ClassLoader may ignore it and unconditionally use the JavaMail provided by the server environment.

Side note: the correct way to loop through an Enumeration is with its hasMoreElements() method:

while (enumer.hasMoreElements()) {
    Header header = (Header) enumer.nextElement();
    System.out.printf("%s: %s%n", header.getName(), header.getValue());
}

Even though using try/catch technically works, it is not correct to use try/catch to detect the end of an Enumeration. It is slower, harder to read, and may inadvertently catch programming errors, which makes debugging considerably harder.